There is instruction for you in cattle. From
the contents of their bellies, from between the dung and blood,
We give you pure milk to drink, easy for drinkers to swallow.
(Qur'an, 16:66)

Say:
“Who provides for you from the heavens and earth?”
Say: “Allah. It is certain that one or the other of
us, either we or you, is following guidance or else clearly
astray.”
(Qur’an, 34:24)

The basic materials which allow the nourishment of the body come
about as a result of chemical changes in the digestive system. These
digested substances then pass through the wall of the intestine
into the blood stream. Due to the circulation of the blood, the
nutriments reach the relevant organs.

Like the other bodily tissues, the milk glands are nourished by
nutriments brought to them by the blood. The blood therefore plays
a most important role in the collection of nutriments from foodstuffs.
Milk is secreted by the milk glands after all these stages and its
nutritional values are particularly high.

Human beings are unable to directly consume either the half-digested
food in an animals stomach or an animals blood. Furthermore, the
direct consumption of these or any of their compounds can lead to
severe illness, and even death. Thanks to the exceedingly complex
biological systems He has created, Allah provides clean and healthy
food for human beings from these fluids.

This is because it forms as a result of digested food being carried
by the blood. High-nutrition milk is thus produced from blood, which
cannot be consumed directly, and semi-digested food.

The formation of milk is by itself an enormous creation miracle.
And it is another miracle altogether that such detailed information
about that formation should be contained in the Quran.

As we have seen, the information about the biological formation
of milk in Surat an-Nahl 66, is in great accordance with the facts
established by modern science. It is quite clear that such information,
requiring detailed knowledge of mammals' digestive systems, could
not have been known at the time when the Qur'an was revealed.